A series of laser microbeam experiments will be conducted on mitotic chromosomes of various cell lines. Specific studies will be undertaken to: (1) elucidate further the possible mechanism(s) of chromosome loss, replacement, and stability; (2) determine the feasibility of using this technique for somatic cell genetic studies and studies on chromosome involvement in carcinogenesis; (3) further investigate the function and organization of the ribosomal genes and the nucleolus. The specific procedures employed in these studies are laser microirradiation, electron microscopy, light microscopy, and tissue culture. BIBLIOGRAPHIC REFERENCES: M. W. Berns, K. Leonardson, and M. Witter. Laser microbeam irradiation of rat kangaroo cells (PTK2) following selective sensitization with bromodeoxyuridine and ethidium bromide. J. Morph. 149: 327-338, 1976. W. M. Berns. A possible two-photon effect in vitro using a focused laser beam. Biophys. J. 16: 973-977, 1976.